1. Field of the Invention
Example embodiments of the present invention relates in general to the field of an unevenness detecting apparatus for compensating for a threshold voltage and method thereof, which includes respective sensor pixel circuit, provided with a plurality of scan lines and a plurality of data lines and arranged on each point which the scan lines and the data lines cross are intersected, and more specifically to an unevenness detecting apparatus and method thereof that can accurately sense a state of a minute unevenness such as a fingerprint by using an active element (e.g., a thin film transistor TFT) as an element which a pixel circuit is constituted.
2. Description of the Related Art
Now, there is a fingerprint sensor as a device configured to sense a shape change of a minute unevenness. The fingerprint sensor used in this system can obtain a shape of a fingerprint as a digital image, and be largely divided into an optical type, a capacitance detection type, a thermal detection type and an ultrasonic detection type sensor. The capacitance detection type sensor among the sensors converts a capacitance change according to a distance between the fingerprint and a sensing electrode into a voltage and outputs the converted voltage as a digital signal.
FIG. 1 is an entire configuration view of an unevenness detecting apparatus configured to sense unevenness such as the fingerprint.
As illustrated in FIG. 1, a general unevenness detecting apparatus is composed of a scan line driving circuit 10 configured to transmit a select signal for selecting respective pixel 40, a pixel circuit 40 arranged on each point which the plurality of scan line 11 connected to a scan line driving circuit 10 and a plurality of data lines 21 are intersected, and a read-out circuit 30 configured to transmit a data value of a sensed voltage to a predetermined IC chip.
FIG. 2 is a circuit view illustrating the pixel circuit 40 included in an unevenness detecting apparatus used in an existing capacitance detecting method.
According to the pixel circuit illustrated in FIG. 2, a capacitance between a sensing electrode (S) which plays a role of capacitor is determined according to a distance between an object such as a fingerprint and a sensing electrode (S). Accordingly, an electric charge charged through a power supply transistor (T2) for a regular time is determined by the capacitance determined according to the distance between the sensing electrode (S) and the object.
If a select signal is applied to a N-th scan line (Gate n) 11 in FIG. 2, a switching transistor (T1) is turned on and transmits the electric charge stored in a capacitor formed between the sensing electrode and the object such as the fingerprint to the data line 21.
However, since a charge distribution is generated by a parasitic capacity of the data line 21 in a process of outputting and moving the electric charge stored in the capacitor as illustrated above, a voltage transmitted to the real read-out IC is decreased.
Accordingly, a function of charge amplification must be performed in the pixel circuit so as to compensate for decrease of the voltage due to the above parasitic capacity. FIG. 3 is a circuit view illustrating a pixel circuit of the capacitance detection type of the existing unevenness detecting apparatus which can perform charge amplification.
Referring to FIG. 3, the change of the capacitance determined according to the distance between the sensing electrode (S) and the object such as the fingerprint changes a gate voltage of a source follow transistor (T3), and if a select voltage is applied to the N-th scan line, a difference between the gate voltage and a threshold voltage of the source follow transistor (T3) is outputted.
The pixel circuit may amplify the electric charge, but change an output signal due to time or generate a fixed pattern noise, according to a spatial non-uniformity of the threshold voltage of the source follow transistor (T3) or a deterioration of characteristics due to time.
Specifically, when a TFT such as an amorphous silicon or a poly crystalline silicon transistor is used, a characteristic of the element is excessively changed. Thus, the above problem may be obvious more and more.